Adsorbent for removing sulfur compound, process for producing hydrogen and fuel cell system

ABSTRACT

The present invention provides an adsorbent for removing sulfur compounds, which adsorbent can effectively remove a variety of sulfur compounds contained in a hydrocarbon fuel to a low concentration even at room temperature; a process for effectively producing hydrogen that can be used in a fuel cell; and a fuel cell system employing hydrogen produced through the process. 
     The adsorbent for removing a sulfur compound contained in a hydrocarbon fuel contains cerium oxide. The process for producing hydrogen that can be used in a fuel cell includes desulfurizing a hydrocarbon fuel through removal of a sulfur compound contained in the a hydrocarbon fuel by use of the aforementioned adsorbent and, subsequently, bringing the thus-desulfurized fuel into contact with a partial-oxidation reforming catalyst, an authothermal reforming catalyst, or a steam reforming catalyst. The fuel cell system employs hydrogen produced through the process.

TECHNICAL FIELD

The present invention relates to an adsorbent for removing sulfurcompounds, to a process for producing hydrogen, and to a fuel cellsystem. More particularly, the invention relates to an adsorbent forremoving sulfur compounds, which adsorbent can effectively remove avariety of sulfur compounds contained in a hydrocarbon fuel to a lowconcentration even at room temperature; to a process for effectivelyproducing hydrogen that can be used in a fuel cell, from the hydrocarbonfuel which has been desulfurized by use of the adsorbent; and to a fuelcell system employing hydrogen produced through the process.

BACKGROUND ART

In recent years, new energy-production techniques have attractedattention from the standpoint of environmental issues, and among thesetechniques a fuel cell has attracted particular interest. The fuel cellconverts chemical energy to electric energy through electrochemicalreaction of hydrogen and oxygen, attaining high energy utilizationefficiency. Therefore, extensive studies have been carried out onrealization of fuel cells for civil use, industrial use, automobile use,etc.

Fuel cells are categorized in accordance with the type of employedelectrolyte, and a phosphate type, a fused carbonate salt type, a solidoxide type, and a solid polymer type have been known. With regard tohydrogen sources, studies have been conducted on methanol; liquefiednatural gas predominantly containing methane; town gas predominantlycontaining natural gas; a synthetic liquid fuel produced from naturalgas serving as a feedstock; and petroleum-derived hydrocarbons such asLPG, naphtha, and kerosene.

When hydrogen is produced from these gas or liquid hydrocarbons, thehydrocarbons are generally partial-oxidation-reformed,autothermal-reformed, or steam-reformed, in the presence of a reformingcatalyst.

When a hydrocarbon fuel such as LPG, town gas, or kerosene is reformedso as to produce hydrogen serving as a fuel, the sulfur content of thehydrocarbon fuel must be reduced to 0.1 ppm or lower in order to preventpoisoning of the reforming catalyst. When a hydrocarbon such aspropylene or butene is employed as a feedstock for petrochemicalproducts, the sulfur content of the hydrocarbon must be reduced to 0.1ppm or lower in order to prevent poisoning of the reforming catalyst.

The aforementioned LPG generally contains sulfur compounds such asmethylmercaptan and carbonyl sulfide (COS), and an odorant such asdimethylsulfide (DMS), t-butylmercaptan (TBM), or methyl ethyl sulfideis intentionally added thereto. Recently, research efforts have beendevoted to utilization of oxygen-containing hydrocarbon compounds, suchas dimethyl ether, as a fuel. Although no sulfur compound is included inthe oxygen-containing hydrocarbon compounds, studies have been conductedon intentional addition of the aforementioned odorant to the hydrocarboncompounds, because the odorant would effectively warn gas leakage.

There have been known a variety of adsorbents which adsorb sulfurcompounds contained in a hydrocarbon fuel such as LPG or town gas so asto remove the compounds from the fuel. Although some of these knownadsorbents exhibit excellent desulfurization performance at about 150 toabout 300° C., currently attained desulfurization performance at ambienttemperature is not satisfactory.

There have been disclosed desulfurizing agents; for example,desulfurizing agents containing hydrophobic zeolite and a metallicelement such as Ag, Cu, Zn, Fe, Co, or Ni carried thereon throughion-exchange (see, for example, Japanese Patent Application Laid-Open(kokai) No. 2001-286753) and desulfurizing agents containing Y-, β-, orX-type zeolite and Ag or Cu carried thereon (see, for example, JapanesePatent Application Laid-Open (kokai) No. 2001-305123). Thesedesulfurizing agents effectively adsorb, at room temperature, mercaptansand sulfides contained in a fuel so as to remove the sulfur compoundsfrom the fuel, but adsorb virtually no carbonyl sulfide.

Copper-zinc desulfurizing agents are also disclosed (see, for example,Japanese Patent Application Laid-Open (kokai) No. 2-302496). Althoughthe desulfurizing agents adsorb a variety of sulfur compounds at 150° C.or higher so as to remove the compounds, sulfur compound adsorptionperformance at 100° C. or lower is unsatisfactory. Also disclosed is adesulfurizing agent containing a porous carrier (e.g., alumina) andcopper carried thereon (see, for example, Japanese Patent ApplicationLaid-Open (kokai) No. 2001-123188). The desulfurizing agent can also beemployed at 100° C. or lower, but its adsorption performance is notsufficient.

DISCLOSURE OF THE INVENTION

Under such circumstances, an object of the present invention is toprovide an adsorbent for removing sulfur compounds, which adsorbent caneffectively remove a variety of sulfur compounds contained in ahydrocarbon fuel to a low concentration even at room temperature.Another object of the invention is to provide a process for effectivelyproducing hydrogen that can be used in a fuel cell, from the hydrocarbonfuel which has been desulfurized by use of the adsorbent. Still anotherobject of the invention is to provide a fuel cell system employinghydrogen produced through the process.

The present inventors have carried out extensive studies in order toattain the aforementioned objects, and have found that cerium oxide,particularly cerium oxide having a mean crystallite size of 10 nm orless, exhibits excellent performance of adsorbing a variety of sulfurcompounds even at ambient temperature, and that hydrogen that can beused in a fuel cell can be effectively produced through reforming of thehydrocarbon fuel which has been desulfurized by use of the adsorbent.The present invention has been accomplished on the basis of thesefindings.

Accordingly, the present invention provides the following.

(1) An adsorbent for removing a sulfur compound contained in ahydrocarbon fuel, characterized in that the adsorbent comprises ceriumoxide.

(2) An adsorbent for removing a sulfur compound as described in (1)above, wherein the adsorbent has a specific surface area of 20 m²/g ormore.

(3) An adsorbent for removing a sulfur compound as described in (1)above, wherein the adsorbent has a specific surface area of 50 m²/g ormore.

(4) An adsorbent for removing a sulfur compound as described in (1)above, wherein the cerium oxide has a mean crystallite size of primaryparticles of 10 nm or less.

(5) An adsorbent for removing a sulfur compound as described in (1)above, wherein the cerium oxide exhibits a cumulative hydrogenconsumption, as calculated up to 600° C. in a temperature-programmedreduction test, of 200 μmol/g or more.

(6) An adsorbent for removing a sulfur compound as described in (1)above, wherein the cerium oxide exhibits a cumulative hydrogenconsumption, as calculated up to 600° C. in a temperature-programmedreduction test, of 300 μmol/g or more.

(7) An adsorbent for removing a sulfur compound as described in (1)above, wherein the adsorbent contains a mixture of cerium oxide and atleast one oxide selected from among Al₂O₃, SiO₂, TiO₂, ZrO₂, and MgO.

(8) An adsorbent for removing a sulfur compound as described in (1)above, wherein the adsorbent further contains at least one elementselected from among the elements belonging to Groups 1 to 15 in theperiodic table, said at least one element being carried on cerium oxide.

(9) An adsorbent for removing a sulfur compound as described in (8)above, wherein the cerium oxide on which at least one element selectedfrom among the elements belonging to Groups 1 to 15 in the periodictable is carried is calcined at 400° C. or lower.

(10) An adsorbent for removing a sulfur compound as described in (8)above, wherein the amount of a carried compound, as reduced to thecorresponding metallic element, of at least one element selected fromamong the elements belonging to Groups 1 to 15 in the periodic table is1 to 90 mass % based on the entire amount of the adsorbent.

(11) An adsorbent for removing a sulfur compound as described in (1)above, wherein the cerium oxide is a complex oxide containing cerium,and at least one metallic element other than cerium selected from amongthe elements belonging to Groups 2 to 16 in the periodic table.

(12) An adsorbent for removing a sulfur compound as described in (1)above, wherein the hydrocarbon fuel is LPG, town gas, natural gas,naphtha, kerosene, gas oil, or at least one hydrocarbon compound oroxygen-containing hydrocarbon compound selected from among ethane,ethylene, propane, propylene, butane, butene, methanol, and dimethylether.

(13) A process for producing hydrogen, characterized in that the processcomprises desulfurizing a hydrocarbon fuel through removal of a sulfurcompound contained in a hydrocarbon fuel by use of an adsorbent asrecited in (1) above and, subsequently, bringing the fuel which has beendesulfurized into contact with a partial-oxidation reforming catalyst,an authothermal reforming catalyst, or a steam reforming catalyst.

(14) A process for producing hydrogen as described in (13) above,wherein the partial-oxidation reforming catalyst, the authothermalreforming catalyst, or the steam reforming catalyst contains rutheniumor nickel.

(15) A process for producing hydrogen for use in a fuel cell asdescribed in (13) above, wherein desulfurizing is performed while nohydrogen or oxygen is added.

(16) A process for producing hydrogen as described in (13) above,wherein the sulfur compound is at least one species selected from amongcarbonyl sulfide, carbon disulfide, hydrogen sulfide, mercaptans,sulfides, and thiophenes.

(17) A process for producing hydrogen as described in (13) above,wherein desulfurizing is performed at 200° C. or lower.

(18) A process for producing hydrogen as described in (13) above,wherein desulfurizing is performed at 100° C. or lower.

(19) A fuel cell system characterized by employing hydrogen producedthrough a process for producing hydrogen as recited in any of (13) to(18) above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an exemplary fuel cell systemaccording to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The adsorbent for removing a sulfur compound of the present inventioncontains cerium oxide and is employed for removing a variety of sulfurcompounds contained in a hydrocarbon fuel.

Examples of the sulfur compound include carbonyl sulfide, carbondisulfide, hydrogen sulfide, sulfur as simple substance, sulfur dioxide,mercaptans, sulfides, and thiophenes.

No particular limitation is imposed on the type of cerium oxidecontained in the adsorbent of the present invention, and examples of theadsorbent include the followings:

(a) an adsorbent solo including cerium oxide or a complex oxidecontaining cerium and an element other than cerium (hereinafter referredto as Ce-M complex oxide);

(b) an adsorbent including a mixture of cerium oxide or a Ce-M complexoxide with another metal oxide;

(c) an adsorbent including a carrier formed of cerium oxide or a Ce-Mcomplex oxide and an active metallic species carried thereon;

(d) an adsorbent including a carrier formed of cerium oxide or a Ce-Mcomplex oxide and another metal oxide, and an active metallic speciescarried thereon;

(e) an adsorbent including a refractory porous carrier and cerium oxideor a Ce-M complex oxide carried thereon; and

(f) an adsorbent including a refractory porous carrier and cerium oxideor a Ce-M complex oxide and an active metallic species carried thereon.

The element other than cerium for forming the aforementioned Ce-Mcomplex oxide is at least one metallic element selected from among thosebelonging to Groups 2 to 16 in the periodic table. Specific examples ofthe Ce-M complex oxide include a Ce—Si complex oxide, a Ce—Zr complexoxide, and a Ce—Si—Zr complex oxide.

In the aforementioned (b) or (d), examples of metal oxides preferablyused in combination with cerium oxide or a Ce-M complex oxide includeoxides of a metal selected from among La, Sc, Y, Nd, Pr, Sm, Gd, and Yb.These metal oxides may be used singly or in combination of two or morespecies.

The active metallic species to be carried on a carrier in the (c) or (d)above may be selected from the elements belonging to Groups 1 to 15 inthe periodic table. Specific examples include Cs, Ba, Yb, Ti, Zr, Hf,Nb, Mo, W, Mn, Re, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Ga,In, Sn, and Bi. These elements may be used singly or in combination oftwo or more species. No particular limitation is imposed on the form ofthese elements, and oxides, metals, and other species may be used. Amongthese elements, Ag, Cu, Ni, Fe, Mn, etc. are preferred for a certaintype and amount of sulfur compound contained in the hydrocarbon fuel.

Through incorporation of the aforementioned active species into acarrier formed of cerium oxide or a Ce-M complex oxide or a carrierformed of cerium oxide or a Ce-M complex oxide and another metal oxide,desulfurization performance of the produced adsorbent can be enhanced.

No particular limitation is imposed on the amount (as reduced tometallic element) of the aforementioned carried active metallic species,and the amount is generally 1 to 90 mass %, preferably 3 to 80 mass %,based on the sum of the active species amount and the carrier amount.

Examples of the refractory porous carrier employable in (e) or (f) aboveinclude silica, alumina, silica-alumina, titania, zirconia, zeolite,magnesia, diatomaceous earth, terra alba, and clay. These species may beused singly or in combination.

An adsorbent formed of any of the above refractory porous carrier andcerium oxide or a Ce-M complex oxide carried thereon is also preferablyemployed in the invention.

The adsorbent of the present invention preferably contains cerium oxidein an amount of 3 mass % or more, more preferably 10 mass % or more,from the viewpoint of desulfurization performance.

The cerium oxide contained in the adsorbent preferably has a meancrystallite size of 10 nm or less, more preferably 1 to 10 nm, from theviewpoint of desulfurization performance. The mean crystallite size ofcerium oxide may be controlled during preparation of the adsorbent.

As used herein, the term “mean crystallite size” of the cerium oxidecontained in the adsorbent refers to a particle size as determined undera transmission electron microscope. The particle is not necessarilycrystalline, and the particle size refers to a primary particle size,regardless of whether the particle is crystalline or amorphous. The term“primary particles” refers to particles which are not aggregated orparticles serving as unit fragments of an aggregate. Even when primaryparticles aggregate to form secondary particles, tertiary particles, orhigher aggregated particles, the mean crystallite size of cerium oxideshould be understood to refer to a primary particle size of ceriumoxide. In the case where the observed particles are not uniform in size,10 or more arbitrary primary particles are selected, and the particlesizes are averaged, thereby providing the mean crystallite size. In thecase where rod-like or needle-like particles are included, each particlesize is obtained from the width (shorter size) of the particle ratherthan from the length (longer size). When cerium oxide is carried on acarrier such as alumina, the mean crystallite size of CeO₂ refers to theprimary particle size of the carried cerium oxide. When cerium oxideforms a solid solution with another oxide, the mean crystallite sizerefers to the primary particle size of the cerium-containing solidsolution particles. In the aforementioned particle size determinationthrough observation under a transmission electron microscope, when theparticles are clearly distinct from one another, the particle size maybe determined by use of an automated counter.

From the viewpoint of desulfurization performance, the aforementionedcerium oxide preferably exhibits a cumulative hydrogen consumption, ascalculated up to 600° C. in a temperature-programmed reduction test(TPR), of 200 μmol/g or more, more preferably 300 μmol/g or more. In thetemperature-programmed reduction test of cerium oxide, a sample (100 mg)was heated to 827° C. at a temperature elevation rate of 10° C./minwhile hydrogen (10% by volume in argon) is introduced at 20 mL/min, andthe cumulative hydrogen consumption up to 600° C. was calculated.

During TPR of cerium oxide, two peaks attributed to reduction (H₂consumption) are observed in a low temperature region (about 300 to 550°C.) and a high temperature region (827° C. or higher), respectively.When cerium oxide is reduced by hydrogen, CeO₂ converts to Ce₂O₃ inaccordance with the following reaction scheme.2CeO₂+H₂→Ce₂O₃+H₂O

Among two TPR peaks, the peak observed in a low temperature region ispossibly attributed to reduction of surface oxygen of the CeO₂particles, whereas the peak observed in a high temperature region ispossibly attributed to reduction of bulk CeO₂. The study by theinventors revealed that the larger the peak in a low temperature region(i.e., hydrogen consumption), the more excellent desulfurizationperformance of cerium oxide at ambient temperature. Although the reasonhas not been completely elucidated, one possible reason is that oxygenatoms of cerium oxide which are reacted with H₂ at about 300 to 550° C.(i.e., reduced) react with a sulfur compound at ambient temperature,thereby chemically adsorbing the sulfur compound.

The adsorbent of the present invention preferably has a specific surfacearea of 20 m²/g or more, more preferably 50 m²/g or more, from theviewpoint of desulfurization performance. The specific surface area ofthe adsorbent may be determined by use of, for example, a specificsurface area measurement apparatus (Product of Yuasa Ionics Inc.)through the following procedure.

Specifically, a sample (about 100 mg) is charged into a sample tube and,as a preliminary treatment, dehydrated by heating at 200° C. for 20minutes under nitrogen flow. Subsequently, the dehydrated sample isbrought into contact with flow of a mixture gas (nitrogen (30%)/helium(70%)) at liquid nitrogen temperature, so as to cause the sample toadsorb nitrogen. Then the adsorbed nitrogen is desorbed, and the amountof nitrogen desorbed is determined by means of a TCD detector, therebydetermining the specific surface area of the sample.

The adsorbent of the present invention may be produced through thefollowing procedure. In the case where the adsorbent is exclusivelyformed from cerium oxide, an aqueous solution containing a ceriumsources (e.g., cerium nitrate) is brought into contact with an aqueousalkaline solution, thereby forming precipitations. The precipitatedsolid is separated through filtration, washed with water, and dried atabout 50 to 200° C. The dried product is calcined at about 250 to 500°C., followed by molding (e.g., pelletization or extrusion) andpulverization to a particle size of interest.

Cerium oxide may be carried on a refractory porous carrier through aconventionally known method such as pore-filling, immersion, orvaporization to dryness. In this case, drying temperature is generallyabout 50 to 200° C., and calcination temperature is generally about 250to 500° C.

Similarly, the aforementioned active metallic species may be carried ona carrier formed of cerium oxide or a similar compound, also through theaforementioned conventionally known method such as pore-filling,immersion, or vaporization to dryness. In this case, drying temperatureis generally about 50 to 200° C., and calcination temperature ispreferably 400° C. or lower, more preferably 100 to 400° C.

The thus-produced adsorbent of the present invention for removing asulfur compound is employed for desulfurization of a hydrocarbon fuel.Examples of the hydrocarbon fuel include LPG; town gas; natural gas;naphtha; kerosene; gas oil; hydrocarbon compounds selected from amongethane, ethylene, propane, propylene, butane, and butene, andoxygen-containing hydrocarbon compounds. The oxygen-containinghydrocarbon compound may be at least one species selected from amongalcohols such as methanol, ethanol, and isopropanol; and ethers such asdimethyl ether and methyl ethyl ether. Of these, dimethyl ether isparticularly preferred.

The sulfur compound concentration of the hydrocarbon-fuel-containing gasto be treated with the adsorbent of the present invention is preferably0.001 to 10,000 ppm by volume, particularly preferably 0.1 to 100 ppm byvolume. Desulfurization is generally performed at a temperature of −50to 200° C. and a GHSV (gas hourly space velocity) of 100 to 1,000,000h⁻¹.

When the desulfurization temperature is higher than 200° C., sulfurcompounds are not easily adsorbed. Thus, the desulfurization temperatureis preferably −50 to 120° C., more preferably −20 to 100° C., and theGHSV is preferably 100 to 100,000 h⁻¹, more preferably 100 to 50,000h⁻¹.

When the desulfurization is performed in the presence of hydrogen, someloaded metallic species may enhance desulfurization performance.However, addition of hydrogen is not essential during desulfurization inthe presence of the catalyst of the present invention. Addition ofoxygen should be avoided so as to prevent possible combustion(oxidation) of a hydrocarbon fuel.

According to the process of the present invention for producing hydrogenthat can be used in a fuel cell, a hydrocarbon fuel is desulfurized byuse of the aforementioned adsorbent of the invention so as to removesulfur compounds, and the desulfurized fuel is reformed, therebyproducing hydrogen.

The reforming may be performed by partial-oxidation reforming,autothermal reforming, or steam reforming. Upon the reforming, thedesulfurized hydrocarbon fuel preferably has a sulfur compoundconcentration of 0.1 ppm by volume or less, particularly preferably 0.05ppm by volume or less, from the viewpoint of the service life of eachreforming catalyst.

The partial-oxidation reforming is a process for producing hydrogenthrough partial oxidation of hydrocarbon in the presence of apartial-oxidation reforming catalyst. The conditions generally employedare as follows: reaction pressure of ambient pressure to 5 MPa, reactiontemperature of 400 to 1,100° C., GHSV of 1,000 to 100,000 h⁻¹, andoxygen (O₂) /carbon ratio of 0.2 to 0.8.

The autothermal reforming is a combination process of partial-oxidationreforming and steam reforming in the presence of an autothermalreforming catalyst. The conditions generally employed are as follows:reaction pressure of ambient pressure to 5 MPa, reaction temperature of400 to 1,100° C., oxygen (O₂) /carbon ratio of 0.1 to 1, steam/carbonratio of 0.1 to 10, and GHSV of 1,000 to 100,000 h⁻¹.

The steam reforming is a process for producing hydrogen through causinghydrocarbon into contact with steam in the presence of a steam reformingcatalyst. The conditions generally employed are as follows: reactionpressure of ambient pressure to 3 MPa, reaction temperature of 200 to900° C., steam/carbon ratio of 1.5 to 10, and GHSV of 1,000 to 100,000h⁻¹.

In the present invention, the aforementioned partial-oxidation reformingcatalyst, autothermal reforming catalyst, and steam reforming catalystmay be appropriately selected from conventionally known catalysts. Amongthem, a ruthenium-containing catalyst and a nickel-containing catalystare particularly preferred. Examples of preferred carrier for thecatalysts include at lease one species selected from among manganeseoxide, cerium oxide, and zirconia. The carrier may be exclusively formedof a metal oxide. Thus, any of the aforementioned metal oxides may beincorporated into a refractory porous inorganic oxide such as alumina,thereby serving as a carrier.

The present invention also provides a fuel cell system employinghydrogen produced through the aforementioned process. The fuel cellsystem of the present invention will next be described with reference tothe attached FIG. 1.

FIG. 1 shows a schematic diagram of an exemplary fuel cell systemaccording to the present invention. As shown in FIG. 1, a fuel containedin a fuel tank 21 is fed to a desulfurizer 23 through a fuel pump 22.The adsorbent of the present invention may be put into the desulfurizer.The fuel which has been desulfrized by the desulfurizer 23 is mingledwith water fed from a water tank through a water pump 24, and the fuelmixture is fed to a gasifier 1 so as to gasify the mixture. The fuelmixture gas is mixed with air fed by means of an air blower 35, and thegas is transferred to a reforming apparatus 31.

The aforementioned reforming catalyst has been charged into thereforming apparatus 31. Through any of the aforementioned reformingreactions, hydrogen or synthesis gas is produced from a fuel mixture(gas mixture containing steam, oxygen, and a hydrocarbon fuel or anoxygen-containing hydrocarbon fuel) fed into the reforming apparatus 31.

The thus-produced hydrogen or synthesis gas is transferred to a COconverter 32 and/or a CO-selective oxidizer 33 for reducing the COconcentration so as not to affect the characteristics of the producedfuel cell. Examples of the catalyst used in the CO converter 32 includeiron-chromium catalysts, copper-zinc catalysts, and noble metalcatalysts. Examples of the catalyst used in the CO-selective oxidizer 33include ruthenium catalysts, platinum catalysts, and mixtures thereof.

A fuel cell 34 is a polymer electrolyte fuel cell including a negativeelectrode 34A, a positive electrode 34B, and a polymer electrolyte 34Cprovided therebetween. The hydrogen-rich gas produced through the abovemethod is fed to the negative electrode, while air is fed to thepositive electrode through the air blower 35. If required, these gasesundergo appropriate humidification (by means of a humidifier notillustrated) before introduction to the electrodes.

In the negative electrode, dissociation of hydrogen to proton andelectron occurs, while in the positive electrode reaction of oxygen withelectron and proton to form water occurs, whereby direct current isprovided between the electrodes 34A and 34B. The negative electrode isformed of platinum black, a Pt-on-activated carbon catalyst, a Pt—Rualloy catalyst, etc. The positive electrode is formed of platinum black,a Pt-on-activated carbon catalyst, etc.

When a burner 31A of the reforming apparatus 31 is connected with thenegative electrode 34A, excess hydrogen may be used as a fuel. In aliquid/gas separator 36 connected with the positive electrode 34B, adischarge gas is separated from water which has been formed from oxygenand hydrogen contained in air fed to the positive electrode 34B. Theseparated water may be use for forming steam.

Notably, since the fuel cell 34 generates heat during electric powergeneration, the heat is recovered through provision of an exhausted heatrecovering apparatus 37 so as to effectively use the recovered heat. Theexhausted heat recovering apparatus 37 includes a heat-exchanger 37A forabsorbing heat generated during reaction; a heat-exchanger 37B fortransferring the heat absorbed in the heat exchanger 37A to water; acooler 37C, and a pump 37D for circulating a cooling medium to theheat-exchangers 37A and 37B and the cooler 37C. Hot water obtained inthe heat exchanger 37B may be effectively used in other facilities.

EXAMPLES

The present invention will next be described in more detail by way ofexamples, which should not be construed as limiting the inventionthereto.

Average crystallite size of cerium oxide, cumulative H₂ consumptiondetermined by the TRP test (≦600° C.), and specific surface area of thedesulfurizing agents produced in the Examples were determined inaccordance with the methods described in the present specification.

Example 1

To a solution of cerium nitrate hexahydrate (special reagent grade,product of Wako Pure Chemical Industries, Ltd.) (470 g) dissolved inion-exchanged water (water which is purified with ion-exchange membrane,1 L) heated at 50° C., an aqueous solution of NaOH (3 mol/L) was addeddropwise under stirring so as to adjust the pH of the mixed solution to13. The mixed solution was further stirred for one hour at a constanttemperature of 50° C.

Subsequently, the formed solid was separated through filtration, washedwith ion-exchanged water (20 L), and the washed product was dried at110° C. for 12 hours in an oven having blower. The dried product wascalcined at 350° C. for three hours. The calcined product was pelletizedand pulverized, thereby producing an adsorbent for removing sulfurcompounds (hereinafter referred to simply as desulfurizing agent) formedof CeO₂ (A) having a particle size of 0.5 to 1.0 mm. The properties ofthe desulfurizing agent are shown in Table 1.

Example 2

To a solution of cerium nitrate hexahydrate (special reagent grade,product of Wako Pure Chemical Industries, Ltd.) (470 g) dissolved inion-exchanged water (1 L) heated at 50° C., ammonia solution (30 mass %)was added dropwise under stirring so as to adjust the pH of the mixedsolution to 12. The mixed solution was further stirred for one hour at aconstant temperature of 50° C.

Subsequently, the formed solid was separated through filtration, washedwith ion-exchanged water (20 L), and the washed product was dried at110° C. for 12 hours in an oven having blower. The dried product wascalcined at 350° C. for three hours. The calcined product was pelletizedand pulverized, thereby producing a desulfurizing agent formed of CeO₂(B) having a particle size of 0.5 to 1.0 mm. The properties of thedesulfurizing agent are shown in Table 1.

Example 3

Cerium nitrate hexahydrate (special reagent grade, product of Wako PureChemical Industries, Ltd.) (605 g) and zirconyl nitrate dihydrate(special reagent grade, product of Wako Pure Chemical Industries, Ltd.)(52.0 g) were dissolved in ion-exchanged water (1 L) heated at 50° C.,thereby producing a preparation liquid A. A 3N NaOH solution wasseparately prepared to serve as a preparation liquid B. The preparationliquid B was added dropwise to the preparation liquid A under stirringso as to adjust the pH of the mixed solution to 13.9. The mixed solutionwas further stirred for one hour at a constant temperature of 50° C.

Subsequently, the formed solid was washed with ion-exchanged water,separated through filtration, and the separated product was dried at110° C. for 12 hours in an oven having blower. The dried product wascalcined at 400° C. for three hours. The calcined product was pelletizedand pulverized, thereby producing a desulfurizing agent formed of amixture of CeO₂ and ZrO₂ (50:50 by mass, CeO₂ (50)-ZrO₂ (50)) having amean particle size of 0.8 mm. The properties of the desulfurizing agentare shown in Table 1.

Example 4

Cerium nitrate hexahydrate (special reagent grade, product of Wako PureChemical Industries, Ltd.) (310 g) was dissolved in ion-exchanged water(60 mL) heated at 50° C. To the solution, alumina (KHD-24) (400 g)wasadded so as to impregnate alumina with the solution. Thesolution-impregnated alumina was dried at 110° C. for 12 hours in anoven having blower. The dried product was calcined at 400° C. for threehours. The calcined product was pelletized and pulverized, therebyproducing a desulfurizing agent formed of CeO₂ (20)/Al₂O₃ (80) (CeO₂ (20parts by mass) is carried by Al₂O₃ carrier (80 parts by mass)) having aparticle size of 0.5 to 1.0 mm. The properties of the desulfurizingagent are shown in Table 1.

Comparative Example 1

The CeO₂ (B) produced in Example 2 was placed in a muffle furnace, andcalcined at 800° C. for six hours, thereby producing a desulfurizingagent formed of CeO₂ (C). The properties of the desulfurizing agent areshown in Table 1.

Comparative Examples 2 to 6

Commercial products of MnO₂, ZnO, alumina, β-type zeolite, and activatedcarbon were employed as desulfurizing agents of Comparative Examples 2to 6, respectively. Specific surface area of these desulfurizing agentsare shown in Table 1.

Test Example 1

Each of the desulfurizing agents produced in Examples 1 to 4 andComparative Examples 1 to 6 was molded and pelletized to a particle sizeof 0.5 to 1 mm. The desulfurizing agent (1 cm³) was packed into adesulfurization tube (inner diameter: 9 mm). COS, dimethyl sulfide(DMS), t-butylmercaptan (TBM), and dimethyl disulfide (DMDS) (each 10ppm by volume, total 40 ppm by volume) were incorporated into propanegas, and the sulfur-compounds-containing gas was caused to flow throughthe tube under the conditions: a desulfuruzing agent temperature of 20°C., ambient pressure, and a GHSV (gas hourly space velocity) of 30,000h⁻¹.

Each sulfur compound concentration at the outlet of the desulfurizationtube was determined hourly by means of a gas chromatograph equipped withan SCD (sulfur chemiluminescence detector). The time at which the sulfurcompound concentration exceeded 0.1 ppm by volume and the total amountof adsorbed sulfur are shown in Table 2.

TABLE 1 CeO₂ Specific H₂ surface area consumption Component of of Mean[≦600° C.] Desulfurizing desulfurizing crystallite (μmol/g- agent agent(m²/g) size (nm) CeO₂) Ex. 1 CeO₂ (A) 147 5 466 Ex. 2 CeO₂ (B) 121 3 326Comp. CeO₂ (C) 21 20-100 102 Ex. 1 Ex. 3 CeO₂ (50)-ZrO₂ (50) 125 102,540 Ex. 4 CeO₂ (20)/Al₂O₃ (80) 204 10 1,665 Comp. MnO₂ 301 Ex. 2 Comp.ZnO 18 Ex. 3 Comp. Alumina 271 (6) * Ex. 4 Comp. β-Type zeolite 638 Ex.5 Comp. Activated carbon 1,080 Ex. 6 * (): value per gram of alumina

TABLE 2 Time at which Total amount of concentration adsorbed sulfurexceeded 0.1 ppm (h) (S g/mL) Ex. 1 5 1.42 Ex. 2 5 1.03 Comp. Ex. 1 20.70 Ex. 3 3 0.95 Ex. 4 2 0.62 Comp. Ex. 2 0 0.11 Comp. Ex. 3 0 0.19Comp. Ex. 4 0 0.38 Comp. Ex. 5 0 0.53

As is clear from Tables 1 and 2, the desulfurizing agents of ComparativeExamples formed of porous material are difficult to lower the sulfurcompound concentration to less than 0.1 ppm. However, the desulfurizingagents of Examples containing a specific cerium oxide exhibit remarkabledesulfurization performance.

Examples 5 to 10

Cerium oxide (A) was impregnated with a salt of each of the metals shownin Table 3, followed by drying at 120° C. and calcining at 400° C.,thereby producing a desulfurizing agent containing a carried metalelement (shown in Table 3) in an amount of 10 mass % with respect to theentire amount of the desulfurizing agent.

Comparative Example 7

A desulfurizing agent was produced from β-type zeolite and Ag carriedthereon in an amount of 10 mass % with respect to the entire amount ofthe desulfurizing agent.

Test Example 2

Each of the desulfurizing agents produced in Examples 5 to 10 andComparative Example 7 was molded and pelletized to a particle size of0.5 to 1 mm. The desulfurizing agent (1 cm³) was packed into adesulfurization tube (inner diameter: 9 mm). Propane gas containing COS(40 vol. ppm) was caused to flow through the tube under the conditions:a desulfuruzing agent temperature of 20° C., ambient pressure, and aGHSV of 30,000 h⁻¹.

COS concentration at the outlet of the desulfurization tube wasdetermined hourly by means of a gas chromatograph equipped with an SCD(sulfur chemiluminescence detector). The time at which the COSconcentration exceeded 0.1 ppm by volume is shown in Table 3.

TABLE 3 Time at which COS Metal loaded concentration Type of Amountexceed 0.1 carrier Metal (mass %) vol ppm (h) Ex. 5 CeO₂ (A) — —  9 Ex.6 CeO₂ (A) Ag 10 15 Ex. 7 CeO₂ (A) Cu 10 11 Ex. 8 CeO₂ (A) Ni 10  15<Ex. 9 CeO₂ (A) Fe 10 15 Ex. 10 CeO₂ (A) Mn 10  15< Comp. Ex. 7 β-zeoliteAg 10  0

As is clear from Table 3, carbonyl sulfide is effectively removed byNi/CeO₂ and Mn/CeO₂.

Examples 11 to 15

Cerium oxide (A) was impregnated with a silver nitride solution so as tocause Ag to be loaded on cerium oxide in an amount of 10 mass % withrespect to the entire amount. The Ag-containing cerium oxide was dried(calcined) at 120° C. and further calcined at each temperature shown inTable 4, thereby producing a desulfurizing agent.

Test Example 3

Each of the desulfurizing agents produced in Examples 11 to 15 wasmolded and pelletized to a particle size of 0.5 to 1 mm. Thedesulfurizing agent (1 cm³) was packed into a desulfurization tube(inner diameter: 9 mm). Propane gas containing dimethyl sulfide (DMS)(40 vol. ppm) was caused to flow through the tube under the conditions:a desulfuruzing agent temperature of 20° C., ambient pressure, and aGHSV of 30,000 h⁻¹.

Dimethyl sulfide concentration at the outlet of the desulfurization tubewas determined hourly by means of a gas chromatograph equipped with anSCD (sulfur chemiluminescence detector). The time at which the dimethylsulfide concentration exceeded 0.1 ppm by volume is shown in Table 4.

TABLE 4 Time at which DMS Metal loaded Calcination concentration Type ofAmount temp. exceed 0.1 carrier Metal (mass %) (° C.) vol ppm (h) Ex. 11CeO₂ (A) Ag 10 500 6 Ex. 12 CeO₂ (A) Ag 10 400 10 Ex. 13 CeO₂ (A) Ag 10300 12 Ex. 14 CeO₂ (A) Ag 10 200 13 Ex. 15 CeO₂ (A) Ag 10 120 14

As is clear from Table 4, within a calcination temperature range of 120to 500° C., the lower the calcination temperature, the longer the timefor causing the dimethyl sulfide concentration to exceed 0.1 vol. ppm.The tendency indicates that the amount of adsorbed sulfur increases asthe calcination temperature is lowered.

INDUSTRIAL APPLICABILITY

The present invention can provide an adsorbent for removal of sulfurcompounds, which adsorbent can effectively remove sulfur compoundscontained in a hydrocarbon fuel to a low concentration even at roomtemperature; a process for effectively producing hydrogen that can beused in a fuel cell from the hydrocarbon fuel which has beendesulfurized by use of the adsorbent; and a fuel cell system employinghydrogen produced through the process.

1. A process for producing hydrogen, comprising: desulfurizing ahydrocarbon fuel by contacting the hydrocarbon fuel to an adsorbentcomprising cerium oxide, primary particles of the cerium oxide having amean crystallite size of 10 nm or less; and subsequently reforming thedesulfurized fuel by bringing the desulfurized fuel into contact with acatalyst comprising at least one member selected from the groupconsisting of a partial-oxidation reforming catalyst, an autothermalreforming catalyst, and a steam reforming catalyst; wherein: neitherhydrogen nor oxygen is added while desulfurizing the hydrocarbon fuel;and the cerium oxide is a cerium oxide that has been calcined at atemperature of from 120 to 400° C., the cerium oxide being selected fromthe group consisting of cerium oxide alone, cerium oxide carried on aporous refractory carrier comprising alumina or zirconia, and ceriumoxide carrying at least one metal selected from the group consisting ofsilver, copper, nickel, iron and manganese.
 2. The process of claim 1,wherein the catalyst comprises at least one member selected from thegroup consisting of ruthenium and nickel.
 3. The process of claim 1,wherein desulfurizing comprises removing at least one sulfur compoundselected from the group consisting of carbonyl sulfide, carbondisulfide, hydrogen sulfide, mercaptans, sulfides, and thiophenes. 4.The process of claim 1, wherein the adsorbent has a specific surfacearea of 20 m²/g or more.
 5. The process of claim 1, wherein theadsorbent has a specific surface area of 50 m²/g or more.
 6. The processof claim 1, wherein the cerium oxide exhibits a cumulative hydrogenconsumption, as calculated up to 600° C. in a temperature-programmedreduction test, of 200 μmol/g or more.
 7. The process of claim 1,wherein the cerium oxide exhibits a cumulative hydrogen consumption, ascalculated up to 600° C. in a temperature-programmed reduction test, of300 μmol/g or more.
 8. The process of claim 1, wherein the at least onemetal is present in an amount of from 1 to 90 mass % based on a totalmass of the adsorbent.
 9. The process of claim 1, wherein thehydrocarbon fuel is selected from the group consisting of LPG, town gas,natural gas, naphtha, kerosene, gas oil, ethane, ethylene, propane,propylene, butane, butene, methanol, and dimethyl ether.
 10. A fuel cellsystem, comprising: a desulfurizer; and a reforming apparatus; wherein:the desulfurizer comprises an adsorbent comprising cerium oxide havingprimary particles with a mean crystallite size of 10 nm or less, thedesulfurizer being configured so that a hydrocarbon fuel can becontacted to the adsorbent; and the reforming apparatus comprises acatalyst comprising at least one member selected from the groupconsisting of a partial-oxidation reforming catalyst, an autothermalreforming catalyst, and a steam reforming catalyst, the reformingapparatus being configured so that a desulfurized fuel can be contactedto the catalyst; wherein: the system is configured so that neitherhydrogen nor oxygen is added when desulfurization is performed; thecerium oxide is a cerium oxide that has been calcined at a temperatureof from 120 to 400° C.; and the cerium oxide is selected from the groupconsisting of cerium oxide alone, cerium oxide carried on a porousrefractory carrier comprising alumina or zirconia, and cerium oxidecarrying at least one metal selected from the group consisting ofsilver, copper, nickel, iron and manganese.